In recent years, since an organic thin film solar cell which is made of an organic photoelectric element can be formed by a coating method, it hat been attracted attention as a solar cell suitable for mass production, and intensive investigation has been made in many research institutions.
The organic thin film solar cell was improved in a charge separation efficiency which had been a problem by adopting what is called a bulk hetero junction structure in which an electronic donor material and an electronic acceptor material were mixed (for example, refer to Patent document 1). In recent years, the photoelectric conversion efficiency was improved to achieve an extent of 5 to 6%, and it can be said that the investigations towards a practical application has been activated more. However, in the organic photoelectric conversion element towards a future practical application, development of the organic photoelectric conversion element which generates electricity at higher efficiency is desired.
Moreover, when manufacturing an organic thin film solar cell with a coating process using a flexible substrate, it is possible to manufacture an inexpensive solar cell with a low manufacturing cost which cannot be realized with the conventional silicone system solar cell by applying coating process so called a roll to roll process. However, for the purpose of practical realization, development of an inexpensive and highly efficient flexible transparent electrode substrate is one of the key investigational works.
Conventionally, as a transparent electrode, there has been mainly used an ITO transparent electrode having an indium-tin complex oxide (ITO) membrane produced by a vacuum deposition method or a sputtering process on transparent base materials, such as glass and a transparent plastic film. However, it was problems that the transparent electrode formed using a vacuum deposition method or sputtering process has inferior manufacturing efficiency, and the manufacturing cost is high and that it is inapplicable to the device application in which a flexible property is required since it is inferior to flexibility.
As a production method of a transparent electrode excellent in manufacturing efficiency, there is disclosed a technique using a conductive fiber like a carbon nanotube (CNT) or a metal nanowire. Here, fixing a part of a conductive fiber is fixed to a substrate with a transparent resin layer, and a part of the conductive fiber is projected on the surface of the transparent resin layer to result in forming an electrode (for example, refer to Patent documents 2 and 3). However, since the electrode of such composition has electro conductivity only in the part from which the conductive fiber is projected on the surface, it does not have a function as a flat electrode. In addition, since the conductive fiber is projected on the surface and too much resistance will be loaded on the projected part, the deterioration from a projected part will occur and this method had the problem that the durability of the whole element was inferior.    Patent document 1: U.S. Pat. No. 5,331,183    Patent document 2: Japanese Translation of PCT International Application Publication No. 2006-519712    Patent document 3: US 2007/0074316